Electroluminescent switching circuit



United States Patent 3,280,341 ELECTROLUMINESCENT SWITCHING CIRCUIT Wilbur E. Du Vail, Gardena, Califl, assignor to The W. W. Henry Company, Huntington Park, Califi, a corporation of California Filed Mar. 11, 1363, Ser. No. 264,453 3 Claims. '(Cl. 30788.5)

This invention relates to electronic switching circuits and more particularly to improvements therein.

Electroluminescent devices have been favorably received as display devices. One of the arrangements employed for displaying different numbers or letters, is to deposit, as by silk screen, a pattern of electroluminescent material on a substrate which pattern comprises seven different and isolated elements which define a sort of square looking number eight. By applying or withholding voltages predetermined ones of these elements are made to glow, whereby a number or letter shape may be defined.

For best results it is recommended that at least 250 volts R.M.S. 'at a frequency between 400 to 1000 cycles be applied to these segments in order to make them glow with a readily visible luminescence. It is not easy to switch voltages of this magnitude having frequencies on the order indicated. This would also tend to negative the use of transistors for this purpose. Various complex systems have been built up to accomplish this purpose. Besides their complications they are expensive. Another factor involved in the use of electroluminescent devices is the requirement for memory. By that is meant, it is desired to maintain a display after the signal specifying it has terminated for at least the period required to register on a human observer, or until there is a change in the display. This requires that some storage facility along with the switching be provided whereby the result desired may be achieved. The requirement for storage has also increased the complexity and cost of the switching circuits used with electroluminescent panels.

Accordingly, an object of this invention is to provide a switching system for the purpose described which is simple and inexpensive.

Another object of the present invention is to provide a switching system with memory for the purpose described which is simple and inexpensive.

Yet another object of the present invention is the provision of a novel and unique switching system.

These and other objects of this invention may be achieved in a switching arrangement wherein each one of the elements on an electroluminescent panel is connected to a switching circuit which comprises a transformer having a center tapped primary and a secondary winding connected to the electroluminescent element. The transformer is a step up transformer which steps up a low voltage signal to the required voltage. Each end of the primary winding is connected to a similarly poled diode. The center tap of the primary winding is connected to the collector of a transistor. The source of excitation power is connected to the other side of these two diodes. The transistor is biased to be normally nonconducting. As result, no voltage appears across the secondary winding, since current flow through the primary winding must occur through one or the other of the diodes and then through the center tap and the transistor connected thereto, back to the source of potential. Means are provided for rendering the transistor conductive when it is desired to permit the electroluminescent element to fiuoresce. In order to provide the required memory a tunnel diode is connected between the base and emitter of the transistor. The tunnel diode is triggered into its high voltage state which renders the transistor conductive. It will retain 3,280,341 Patented Oct. 18, 1966 this state whereby the transistor is maintained conductive even though the initial switching signal has been removed. A reset signal is required to restore the tunnel diode to its low voltage state whereby the transistor is no longer conductive and power is no longer applied to the electroluminescent element.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself both as to its organization and method of operation as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings, in which:

FIGURE 1 is a block diagram illustrating the use of the embodiment of the invention;

FIGURE 2 is a circuit diagram of an embodiment of the invention.

Referring now to FIGURE 1 there may be seen a block diagram illustrating how the embodiment of the invention may be employed with an electroluminescent panel 10. This panel has deposited thereon an arrangement of electroluminescent elements respectively 12A through 12G. These elements are arranged in the form of a somewhat square figure eight. The seven elements are electrically insulated from one another, being deposited on a common dielectric beneath which is usually a common transparent conductive layer, beneath which is the substrate which is normally used. A source of energy designated as a 400 cycle source 14, has one side connected to the electroluminescent layer and the other side connected to a different one of the switching circuits respectively 16A through 16G. These switch circuits are normally biased off. The ones of these switch circuits which are turned on in order to allow power from the 400 cycle source to be applied to the predetermined sectors, is determined by a switch control matrix 18. The switch control matrix selective applies enabling signals to the respective gates 20A through 206. The outputs of these gates are applied to the respective switch circuits 16A through 16G. A source of strobing signals designated as a strobe source 22, opens the gates at a predetermined time whereby the switch circuit connected to these open gates, are enabled to apply power to the predetermined elements of the electroluminescent material to cause them to fluoresce. The strobe source is employed where it is desired to turn on all elements at once.

The arrangement shown in FIGURE 1 is fairly conventional. The switch control matrix may be a diode or transistor matrix to which addressing signals for selecting the predetermined ones of the electroluminescent elements for fluorescing are applied. The gate circuits and the strobe may also be conventional arrangements. Each one of the switching circuits, however, in accordance with this invention, are represented by the circuits shown in FIGURE 2. The 400 cycle source 14, feeds an output transformer 30, having a center tapped secondary winding 32. This secondary winding is connected to all of the switch circuits 16A through 16G. Each one of the switching circuits will have a pair of diodes 34, 36, which are similarly poled and which have their cathodes connected to the ends of the secondary winding 32. Another transformer 38, has a primary winding 381 which is center tapped and a secondary winding 388. The voltage applied from the transformer 30 to the primary winding 38F through the diodes 34, 36, should be on the order of the voltage required for operating a transistor. The transformer 38 is a step up transformer and steps up this voltage to the value required for operating an electroluminescent panel. In an embodiment of the invention which was built, the voltage which was applied to the primary winding 38P was on the order of 20 volts R.M.S. and

3 the voltage which appeared on the secondary winding was on the order of 250 volts R.M.S.

The ends of the primary winding 33? are connected to the anodes of the diodes 34, 36. The center tap of the winding 381 is connected to the collector of a transistor 40. The emitter of the transistor is connected to ground. Between the base and the emitter of the transistor there is connected a tunnel diode 4 -2. A capacitor 44, is connected across the tunnel diode in order to reduce its transient response. The transistor 49 is biased to be nonconductive by having the base connected through a resistor 46 to a biasing potential source 48. The base of the transistor 40 is also connected to the collector of a gating transistor 50. The base of the gating transistor is connected to the strobe source and the emitter of the gating transistor is connected to the switch control matrix 18. The base of the transistor 40 is also connected to a reset pulse source 52.

It should be noted that because of the presence of the diodes 34, 36, any voltage which is delivered to the transistor 40 is a half wave rectified voltage. Therefore, the voltage is suitable for utilization by a transistor. However, no current will flow through either half of the primary winding 38?, and therefore no current will be induced in the secondary winding 388, when transistor 49 is biased noncond-uctive. The current path through the transformer winding must fiow through transistor 40. As soon as the transistor 50 applies an enabling signal to the base of transistor 46 it is rendered conductive whereby current can flow through the primary winding 38F to induce a voltage on the secondary winding 388.

The tunnel diode 42, when transistor 40 .is in its nonconducting state, remains in the stability state wherein it has low conductivity. The same signal which is applied from transistor 50 to the base of transistor 46 drives the tunnel diode to its second stability state whereby it assumes a high state of conductivity. As a result, the base of the transistor 40 is brought to substantially the potential of the emitter and the transistor 40 is maintained conductive. Thus, the tunnel diode 42 constitutes a form of memory which will maintain the electroluminescent element which has been energized fluorescing even though the signal which rendered transistor 40 conductive has been removed. This condition will remain until a negative signal is applied from the reset pulse source 52, whereby the tunnel diode is returned to its low conductive state and transistor 40 is biased oft. Thereby power is removed from the fluorescing one of the electroluminescent elements. If it is desired not to have the memory feature but as long as the signal which turned it on is present, then the tunnel diode 42 and the capacitor 44 may be eliminated. Also, if preferred, the base of the transistor 40 may be directly connected to an output of the switch control matrix to assure this type of operation. The transistor 4% is of the pnp type and therefore the diodes 34 are poled as shown, with anodes connected to transformer 38 and cathodes to transformer 30. 'If transistor 40 were of the pnp type the diodes would be reversed.

There has accordingly been described and shown herein a novel, useful, simple and more inexpensive circuit than those employed heretofore, for the purpose of operating selectively anelectroluminescent panel for displaying either characters or numbers. The circuit provides the required power switching function while the control function is carried on at a low enough level to use solid state devices.

I claim:

1. A switching system for applying power from a source to a load comprising a first transformer having a primary and center tapped secondary winding, means connecting said source to said primary winding, a second transformer having a center tapped primary and secondary winding, means connecting said load to said second transformer secondary winding, a first diode connected between one end of said first transformer secondary winding and one one of said second transformer primary winding, a second diode connected between the other end of said first transformer secondary winding and the other end of said second transformer primary winding, said first and second diodes being identically poled relative to said transformer winding and each other, a transistor having a collector, emitter and base electrode, means connecting said collector to said second transformer primary winding center tap, means connecting said emitter to said first transformer secondary winding center tap, first means for applying signals to said transistor base to render said transistor substantially conductive and thereby apply power to said load, tunnel diode means connected between said transistor base and emitter driven to and maintained in its conductive state for maintaining said transistor conductive responsive to a signal from said first means for applying signals, and second means for applying signals to said transistor base for rendering said transistor and tunnel diode means nonconductive.

2. A switching system for applying power from a source to a load comprising a transformer having a center tapped primary winding and a secondary winding, means connecting said load to said secondary winding, means connecting said source to the ends of said center tapped primary winding including a first and second diode, said first and second diodes being connected with a similar poling to said primary winding, a control transistor having an emitter, base and collector electrode, means connecting said collector to the center tap of said transformer primary winding, means connecting said emitter to said source, means connected to said control transistor base to control the conductive state of said transistor to thereby control the power applied from said source to said load, said means connected to said control transistor base including a tunnel diode connected between said transistor base and emitter, said tunnel diode having a first relatively low conductive state and a second relatively high conductive state, first signal means for driving said tunnel diode from its first to its second conductive state and maintaining it in its second conductive state whereby said transistor is driven and maintained conductive, and second signal means for driving said tunnel diode from its second to its first conductive state to thereby render said transistor substantially noncond-uctive.

3. A switching system as recited in claim 2 wherein said first signal means includes a signal transistor having base, emitter and collector electrodes, said signal transistor collector electrode being connected to said control transistor base, means for applying first signals to said signal transistor base, and means for applying operating potential to said switching transistor base and emitter and across said tunnel diode.

References Cited by the Examiner UNETED STATES PATENTS 1/1965 Fitzwater 30788.5 8/ 1965 Amodei 30788.5 

1. SWITCHIN SYSTEM FOR APPLYING POWER FROM A SOURCE TO A LOAD COMPRISING A FIRST TRANSFORMER HAVING A PRIMARY AND CENTER TAPPED SECONDARY WINDING, MEANS CONNECTING SAID SOURCE TO SAID PRIMARY WINDING, A SECOND TRANSFORMER HAVING A CENTER TAPPED PRIMARY AND A SECONDARY WINDING MEANS CONNECTING SAID LOAD TO SAID SECOND TRANSFORMER SECONDARY WINDING, A FIRST DIODE CONNECTED BETWEEN ONE END OF SAID FIRST TRANSFORMER SECONDARY WINDING AND ONE ONE OF SAID SECOND TRANSFORMER PRIMARY WINDING, A SECOND DIODE CONNECTED BETWEEN THE OTHER END OF SAID FIRST TRANSFORMER SECONDARY WINDING AND THE OTHER END OF SAID SECOND TRANSFORMER PRIMARY WINDING, SAID FIRST AND SECOND DIODES BEING IDENTICALLY POLED RELATIVE TO SAID TRANSFORMER WINDING AND EACH OTHER, A TRANSISTOR A COLLECTOR, EMITTER AND BASE ELECTRODE, MEANS CONNECTING SAID COLLECTOR TO SAID SECOND TRANSFORMER PRIMARY WINDINGS CENTER TAP, MEANS CONNECTING SAID IMITTER TO SAID FIRST TRANSFORMER 